System and method for natural gas liquefaction

ABSTRACT

A system for natural gas liquefaction includes a natural gas source for providing a flow of natural gas and a moisture removal system located downstream of the natural gas source. The system includes a first heat exchanger located downstream of the moisture removal system for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem. The system includes one first throttle valve located downstream of heat exchanger for expanding the flow of natural gas and causing reduction in pressure and temperature of the flow of natural gas. The system includes a filter subassembly for separating solid particles present in the flow of natural gas. The system includes a second heat exchanger located downstream of the filter subassembly and is configured to transfer heat from a natural gas vapor flow path to a second refrigerant flow path of the refrigeration cycle subsystem.

BACKGROUND

The present technology relates generally to liquid natural gas production and, in particular, to systems and methods for natural gas liquefaction and filtration of solid carbon dioxide particles.

Generally, natural gas refers to a methane-rich gas mixture that can include carbon dioxide, nitrogen, hydrogen sulfide, other hydrocarbons, and moisture in various proportions. In at least some known applications, natural gas is used as an alternative to other known fuels such as gasoline and diesel. To be used as an alternative fuel, or to facilitate storage and/or transport, natural gas is typically processed to convert the natural gas into liquefied natural gas (LNG). Typically liquefying natural gas includes cooling the natural gas to about the liquefaction temperature of methane, which is about −161° C. under atmospheric pressure. However, since some commonly found constituents of natural gas (e.g. moisture and carbon dioxide) have higher freezing points than methane, solidification of the constituents may occur when cooled to the liquefaction temperature of methane, thereby forming a LNG-rich slurry. The LNG-rich slurry is generally unsuitable for use as alternative fuel. Impurities freezing in a heat exchanger during natural gas liquefaction also can cause operational problems during LNG production.

Conventional methods of forming purified LNG typically includes removing CO₂ in the raw natural gas before cooling it to the liquefaction temperature of methane. However, known removal systems are costly to implement and generally have a relatively large ecological and/or physical footprint. Other known methods of forming purified LNG include removing the solidified CO₂ from LNG via gravity separation and/or cyclone separation. However, while such removal methods are generally effective at removing relatively large solidified CO₂ particles from the LNG-rich slurry, they are less effective at removing smaller particles

Therefore, the inventors have provided an improved system and method for natural gas liquefaction and filtration of solid carbon dioxide particles.

BRIEF DESCRIPTION

In accordance with an example of the present technology, a system for natural gas liquefaction includes a natural gas source for providing a flow of natural gas. The system also includes a moisture removal system located downstream of the natural gas source and configured to remove moisture from a natural gas flow path that is in fluid communication with the natural gas source. The system further includes a first heat exchanger located downstream of the moisture removal system and configured for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem. Furthermore, the system includes at least one first throttle valve coupled downstream of the first heat exchanger for expanding the flow of natural gas and causing reduction in pressure and temperature of the flow of natural gas and formation of liquid natural gas, solid CO₂ particles and cold vapor. The system also includes a filter subassembly for separating the solid CO₂ particles present in the flow of liquid natural gas. The system also includes a second heat exchanger located downstream of the filter subassembly and is configured to transfer heat from a natural gas vapor flow path to a second refrigerant flow path of the refrigeration cycle subsystem. Further, the system includes a storage tank assembly located downstream of the filter assembly for storing liquefied natural gas.

In accordance with an example of the present technology, a method of liquefying a natural gas includes directing a flow of natural gas from a natural gas source to a moisture removal system for removing moisture. The method also includes directing the flow of natural gas through a first heat exchanger for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem. Further, the method also includes expanding the flow of natural gas passing out of the first heat exchanger via a first throttle valve causing reduction in pressure and temperature of the flow of natural gas and further resulting in liquefaction of the flow of natural gas and CO₂ solidification. Furthermore, the method also includes filtering the flow of natural gas for separating solid particles in a filter subassembly and directing the flow of cold vapor through a second heat exchanger configured transfer heat from transfer heat from to a second refrigerant flow path of the refrigeration cycle subsystem. Finally, the method includes storing the filtered flow of natural gas natural in a storage tank assembly.

In accordance with an example of the technology, a system for natural gas liquefaction includes a natural gas source for providing a flow of natural gas. The system includes a moisture removal system located downstream of the natural gas source and configured to remove moisture from a natural gas flow path that is in fluid communication with the natural gas source. The system also includes a first heat exchanger located downstream of the moisture removal system and configured for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem. Further, the system includes at least one first throttle valve coupled downstream of the heat exchanger for expanding the flow of natural gas and causing reduction in pressure and temperature of the flow of natural gas and CO2 solidification and a filter subassembly for separating solid CO2 particles present in the flow of natural gas. Furthermore, the system includes a storage tank assembly located downstream of the filter assembly for storing liquefied natural gas, wherein the storage tank assembly comprises a heat exchanger integrated with a storage tank.

DRAWINGS

These and other features, aspects, and advantages of the present technology will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 schematically shows an exemplary liquefaction system in accordance with an example of the present technology;

FIG. 2 schematically shows an exemplary liquefaction system in accordance with another example of the present technology;

FIG. 3 schematically shows an exemplary liquefaction system in accordance with yet another example of the present technology;

FIG. 4 is a flow chart of a method of liquefying a natural gas in accordance with an example of the present technology.

DETAILED DESCRIPTION

When introducing elements of various embodiments of the present technology, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters are not exclusive of other parameters of the disclosed examples.

FIG. 1 schematically shows an exemplary liquefaction system 10 in accordance with an example of the present technology. The system 10 includes a natural gas source 12 for providing a flow of natural gas to a moisture removal subsystem 14 located downstream of the natural gas source 12. The moisture removal subsystem 14 is configured to remove moisture from a natural gas flow path 16 that is in fluid communication with the natural gas source 12. The system 10 also includes a first heat exchanger 18 located downstream of the moisture removal subsystem 14. The first heat exchanger 18 is a multi-path heat exchanger configured for exchanging heat between the natural gas flow path 16 and a first refrigerant flow path 20 of a refrigerant cycle subsystem 21.

The first refrigerant flow path 20 includes a refrigerant fluid mixture that is capable of absorbing latent heat from the flow of natural gas in the natural gas flow path 16 through the first heat exchanger 18. Such an absorption of latent heat facilitates a liquefaction of the natural gas. In a non-limiting example, the temperature of the natural gas passing through the first heat exchanger 18 may be reduced from about 101 degree Fahrenheit to about −170 degree Fahrenheit.

In some embodiments, the system 10 also includes at least one first throttle valve 22 coupled downstream of the first heat exchanger for expanding the flow of liquefied natural gas and causing reduction in pressure and temperature of the flow of liquefied natural gas. By causing a sudden drop of temperature of the natural gas, a portion of the liquefied natural gas may turn into vapor. While not intending to be bound by theory, the inventors believe such a mechanism is due to a Joule-Thomson effect. In addition, the sudden temperature drop turns the carbon dioxide present in the natural gas into solid particles and thus, the liquefied natural gas contains carbon dioxide particles downstream of first throttle valve 22.

In some embodiments, the system 10 further includes a filter subassembly 24 for separating the solid particles present in the liquefied natural gas. This filter subassembly 24 is also configured to separate a vapor portion of the natural gas. The system 10 further includes a storage tank assembly 25 located downstream of the filter assembly 24 for storing the liquefied natural gas.

As shown in FIG. 1, the refrigerant cycle subsystem 21 includes a compressor 26 located downstream of the first refrigerant flow path 20 and configured to compress the refrigerant fluid mixture flowing through the first refrigerant flow path 20. By compressing the refrigerant fluid mixture, the refrigerant fluid mixture becomes hotter. The refrigerant fluid mixture may then be passed through an air cooler 28 located downstream of the compressor 26 configured to cool the refrigerant mixture. Cooling the refrigerant mixture allows the refrigerant mixture to reject heat into ambient.

In some embodiments, the refrigerant cycle subsystem 21 may also include a phase separator 30 located downstream of the air cooler 28 for separating a vapor portion from a liquid portion of the refrigerant mixture. The vapor portion of the refrigerant mixture includes a vapor stream composed of species with lower boiling points, e.g., lighter hydrocarbons, while the liquid portion includes a liquid stream having species with higher boiling points, e.g., heavier hydrocarbons.

Further, as shown in FIG. 1, the refrigeration cycle subsystem 21 includes a third refrigerant flow path 32 and a second refrigerant flow path 34 connected to a bottom end of the phase separator and a top end of the phase separator 30 respectively. The third refrigerant flow path 32 carries the liquid portion of the refrigerant mixture while the second refrigerant flow path 34 carries the vapor portion of the refrigerant mixture. Each of the third refrigerant flow path 32 and the second refrigerant flow path 34 is directed through the first heat exchanger 18 for allowing heat transfer to the refrigerant fluid mixture flowing in the first refrigerant flow path 20.

In some embodiments, the refrigeration cycle subsystem 21 also includes a three-way valve 36 located downstream of the phase separator 30 in the second refrigerant flow path 34 that connects with the third refrigerant flow path 32 and the second refrigerant flow path 34. The three-way valve 36 is configured for controlling flow of refrigerant mixture in the second and the third refrigerant flow paths 34, 32. Particularly, the vapor portion of the refrigerant mixture flowing in the second refrigerant flow path 34 is divided into two streams by the three-way valve 36. One vapor stream 33 is combined with the liquid portion of the refrigerant mixture flowing in the third refrigerant flow path 32 and then the combined stream is directed to the first heat exchanger 18 while the remaining vapor stream with lower boiling points in the second refrigerant flow path 34 is also passed to the first heat exchanger 18.

The refrigeration cycle subsystem 21 includes one second throttle valve 38 located in the third refrigerant flow path 32 downstream of the first heat exchanger 18 for further expanding the refrigerant mixture in the third refrigerant flow path 32. This causes the temperature of the refrigerant mixture in the third refrigerant flow path 32 to decrease, thereby causing at least some of the refrigerant mixture to become vapor due to the Joule-Thomson effect. The third refrigerant flow path 32 downstream of the second throttle valve 38 connects with a return flow path of the refrigeration cycle subsystem 21 to form the first refrigeration flow path 20 passing through the first heat exchanger 18. The return flow path carries the refrigerant mixture of the second refrigerant flow path 34 after passing through a plurality of heat exchangers. Thus, the first refrigeration flow path 20 is directed to a cold side of the first heat exchanger 18 to absorb heat from a hot side, thereby, completing the refrigeration cycle. As shown, the refrigeration cycle subsystem 21 includes a second heat exchanger 40 located downstream of the second refrigerant flow 34 path and is configured to transfer heat from the second refrigerant flow path 34 to the return flow path of the refrigeration cycle subsystem 21. This leads to cooling of the vapor portion of the refrigerant mixture flowing in the second refrigerant flow path 34.

Furthermore, the refrigeration cycle subsystem 21 includes one third throttle valve 42 located downstream of the third heat exchanger for expanding the refrigerant mixture flowing in the second refrigerant flow path 34. At the end of the expansion by the third throttle valve 42, the temperature of the refrigerant mixture is further reduced below the temperature of the liquefied natural gas in the storage tank assembly 25. The refrigeration cycle subsystem 21 further includes a second heat exchanger 44 located downstream of the third heat exchanger 40 and is configured to transfer heat from a natural gas vapor flow path 46 to the refrigerant mixture in the second refrigerant flow path 34 of the refrigeration cycle subsystem 21. The natural gas flow path 46 carries the vapor portion of the natural gas after being separated in the filter subassembly 24 from the liquefied natural gas. As the result of the heat transfer in the second heat exchanger 44, the temperature of the refrigerant mixture increases while a majority of the vapor portion of the natural gas vapor is condensed. A cycle 39 is shown in FIG. 1 that represents this refrigeration cycle for recovering the vapor of the natural gas. A first portion of condensed natural gas from the natural gas vapor flow path 46 is directed back to the storage tank assembly 25 and a second portion of non-condensed natural gas is directed to a plurality of devices 48 for further use. In a non-limiting example, the temperature of the first portion of condensed natural gas natural gas may be about −250 degrees Fahrenheit and a pressure of about 25 pounds per square inch. After leaving the second heat exchanger 44, the refrigerant mixture in the second refrigerant flow path 34 is now the return flow path that enters the third heat exchanger 40 to absorb heat from a hot side. The resulting heated mixture in the return flow path is then combined with the refrigerant mixture of the third refrigerant flow path 32 and the resultant combined refrigerant mixture is direct back to the first heat exchanger 18 through the first refrigerant flow path 20 to complete the refrigeration cycle for vapor recovery.

FIG. 2 schematically shows another exemplary liquefaction system 50 in accordance with an example of the present technology. The liquefaction system 50 is similar to the liquefaction system 10 except that an ejector 52 is used to replace the second throttle valve 38. The ejector 52 allows further reducing the pressure of the refrigerant mixture at a hot side of the second refrigerant flow path 34 through the second heat exchanger 44 and thus, a pressure ratio across the third throttle valve 42 is larger for the liquefaction system 50 as compared to the liquefaction system 10. The larger pressure ratio can result in a larger cooling capacity. A suction side of the ejector 52 is connected to a cold side of the third heat exchanger 40 and a motive fluid for the ejector 52 is the refrigerant mixture in the third refrigerant flow path 32 with higher boiling points flowing downstream of the first heat exchanger 18.

FIG. 3 schematically shows another exemplary liquefaction system 60 in accordance with an example of the present technology. In this embodiment, the liquefaction system 60 is similar to the liquefaction system 10 except that a storage tank assembly 62 includes a heat exchanger 64 integrated with a storage tank 66. This liquefaction system 60 offers two advantages over the liquefaction system 10. Firstly, the integration of the liquefaction system 60 reduces the parts and simplifies assembly of the system. Secondly, the heat exchanger 62 located in the storage tank 64 can be used to adjust the pressure of the storage tank 64.

FIG. 4 is a flow chart of a method 100 of liquefying a natural gas in accordance with an example of the present technology. At step 102, the method 100 includes directing a flow of natural gas from a natural gas source to a moisture removal system for removing moisture. At step 104, the method 100 also includes directing the flow of natural gas through a first heat exchanger for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem. Further at step 106, the method 100 includes expanding the flow of natural gas passing out of the first heat exchanger via a first throttle valve causing reduction in pressure and temperature of the flow of natural gas and further resulting in liquefaction of the flow of natural gas. Furthermore, at step 108, the method 100 also includes filtering the flow of natural gas for separating solid particles in a filter subassembly. The filtering of the flow of natural gas may include channeling a slurry including liquefied natural gas and solidified carbon dioxide towards a filter house; collecting the solidified carbon dioxide on a filter element in the filter house to form a flow of purified liquefied natural gas; and directing a pulse of cleaning fluid through the filter element to remove the solidified carbon dioxide therefrom. The cleaning fluid includes at least one of methane and carbon dioxide. Finally the method 100 includes storing the filtered flow of natural gas natural in a storage tank assembly at step 110. The method 100 also includes recycling a refrigerant fluid mixture in the refrigeration cycle subsystem through the first refrigeration flow path, a second refrigeration flow path and a third refrigeration flow path. This includes flowing the refrigeration mixture with lower boiling point temperatures through a third heat exchanger and a second heat exchanger located downstream of the second refrigeration flow path. The method 100 also includes expanding the refrigeration mixture flowing in second refrigeration flow path via a second throttle valve. Further, the method 100 includes expanding the refrigeration mixture flowing in second refrigeration flow path via a third throttle valve located downstream of the third heat exchanger and prior to the second heat exchanger.

Advantageously, the present technology provides a low-cost and less natural gas vapor exiting the refrigeration cycle. The system also enables removal of moisture from the natural gas upstream of the first heat exchanger prior to liquefaction. The present technology also enables removing the solidified constituents such as solid carbon dioxide particles from the liquefied natural gas downstream of the first heat exchanger.

Furthermore, the skilled artisan will recognize the interchangeability of various features from different examples. Similarly, the various methods and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular example. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or improves one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

While only certain features of the technology have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the claimed inventions. 

1. A system for natural gas liquefaction, the system comprising: a natural gas source for providing a flow of natural gas; a moisture removal subsystem located downstream of the natural gas source and configured to remove moisture from a natural gas flow path that is in fluid communication with the natural gas source; a first heat exchanger located downstream of the moisture removal system and configured for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem; at least one first throttle valve coupled downstream of the first heat exchanger for expanding the flow of natural gas and causing reduction in pressure and temperature of the flow of natural gas and formation of liquid natural gas, solid carbon dioxide particles and cold vapor; a filter subassembly for separating solid carbon dioxide particles present in the flow of liquid natural gas; a second heat exchanger located downstream of the filter subassembly and is configured to transfer heat from a natural gas vapor flow path to a second refrigerant flow path of the refrigeration cycle subsystem; and a storage tank assembly located downstream of the filter assembly for storing liquefied natural gas.
 2. The system of claim 1, wherein the refrigerant cycle subsystem comprises a compressor located downstream of the first refrigerant flow path and is configured to compress the refrigerant fluid mixture flowing through the first refrigerant flow path.
 3. The system of claim 2, wherein the refrigerant cycle subsystem comprises an air cooler located downstream of the compressor for cooling the refrigerant mixture passing through the compressor.
 4. The system of claim 1, wherein the refrigerant cycle subsystem comprises a phase separator located downstream of the air cooler for separating a vapor portion from a liquid portion of the refrigerant mixture.
 5. The system of claim 4, wherein the refrigeration cycle subsystem comprises the second refrigerant flow path and a third refrigerant flow path connected to a top end of the phase separator and a bottom end of the phase separator respectively.
 6. The system of claim 5, wherein the refrigeration cycle subsystem comprises an ejector located downstream of the third regrigerant flow path for reducing pressure of the refrigerant mixture flowing in the second refrigerant flow path.
 7. The system of claim 5, wherein the refrigeration cycle subsystem comprises a three-way valve located downstream of the phase separator in the second refrigerant flow path that connects with the third and the second refrigerant flow paths and controls flow of refrigerant mixture in the second and the third refrigerant flow paths.
 8. The system of claim 5, wherein the refrigeration cycle subsystem comprises one second throttle valve located downstream of the first heat exchanger for expanding the refrigerant mixture flowing in the third refrigerant flow path.
 9. The system of claim 5, wherein the third refrigerant flow path downstream of the second throttle valve connects with a return flow path of the refrigeration cycle subsystem to form the first refrigeration flow path passing through the first heat exchanger.
 10. The system of claim 9, wherein the return flow path carries the refrigerant mixture of the second refrigerant flow path after passing through a plurality of heat exchangers.
 11. The system of claim 10, wherein the refrigeration cycle subsystem comprises a third heat exchanger located downstream of the second refrigerant flow path and is configured to transfer heat from the second refrigerant flow path to the return flow path of the refrigeration cycle subsystem.
 12. The system of claim 10, wherein the refrigeration cycle subsystem comprises one third throttle valve located downstream of the third heat exchanger for expanding the refrigerant mixture flowing in the second refrigerant flow path.
 13. The system of claim 1, wherein the storage tank assembly comprises a heat exchanger integrated with a storage tank.
 14. A method of liquifying a natural gas, the method comprising: directing a flow of natural gas from a natural gas source to a moisture removal system for removing moisture; directing the flow of natural gas through a first heat exchanger for exchanging heat between the natural gas flow path and a first refrigerant flow path of a refrigerant cycle subsystem; expanding the flow of natural gas passing out of the first heat exchanger via a first throttle valve causing reduction in pressure and temperature of the flow of natural gas and further resulting in liquefaction of the flow of natural gas and formation of liquid natural gas and a flow of cold vapor; filtering the flow of liquid natural gas for separating solid carbon dioxide particles in a filter subassembly; directing the flow of cold vapor through a second heat exchanger configured transfer heat from transfer heat from to a second refrigerant flow path of the refrigeration cycle subsystem; and storing the filtered flow of natural gas natural in a storage tank assembly.
 15. The method of claim 14, wherein the filtering of the flow of natural gas comprises channeling a slurry including liquefied natural gas and solidified carbon dioxide towards a filter house; collecting the solidified carbon dioxide on a filter element in the filter house to form a flow of purified liquefied natural gas; and directing a pulse of cleaning fluid through the filter element to remove the solidified carbon dioxide therefrom, wherein the cleaning fluid includes at least one of methane and carbon dioxide.
 16. The method of claim 15, further comprising recycling a refrigerant fluid mixture in the refrigeration cycle subsystem through the first refrigeration flow path, a second refrigeration flow path and a third refrigeration flow path.
 17. The method of claim 16, further comprising flowing the refrigeration mixture through a third heat exchanger and a second heat exchanger located downstream of the second refrigeration flow path.
 18. The method of claim 17, further comprising expanding at least one of the refrigeration mixture flowing in the second refrigeration flow path via a second throttle valve or the third refrigeration flow path via a third throttle valve, wherein the third throttle valve is located downstream of the second heat exchanger and prior to the third heat exchanger.
 19. The method of claim 15, further comprising expanding the refrigeration mixture flowing in second refrigeration flow path via a third throttle valve located downstream of the third heat exchanger and prior to the second heat exchanger. 